What is the term for calling off a war when you are winning?
Solution 1:
I would say the winning force "withdrew" from the conflict, or one of its many synonyms: it "bowed out of", "exited", or "abandoned" the conflict, "discontinued" hostilities, "disengaged" from battle, "abjured" the war, "retreated" and therefore "foreswore" or "relinquished" its formal victory, and so on and so forth.
None of these words have a particularly martial connotation (I don't think there exists a martial term which means "forfeit, but not lose") but that's easily addressed through context, as in the examples above.
Or, you could describe it the way my father did the first time I went to Atlantic City: Quit while you're ahead.
Solution 2:
To echo what @Edwin Ashworth said; you cannot end a war without either:
- Killing everyone who you were at war with (e.g. genocide).
- Agreeing to end the war with the other side(s).
The closest I can think of is that one side "having achieved its objectives, no longer prosecuted the war".